Enforce timeouts and add simple http2 client test cases

[TysonAndre]

Enforce a 10 second timeout - if any request takes longer than that,
then assume that APNs is having networking or server issues so that
the node-apn client can disconnect and reconnect in the background.

Enforce a 10 second timeout - If a request is sent but there have been no successful response to any
request made on that connection in the last 10 seconds, then assume the connection is dead and terminate the connection.

There's probably a better way to do this, e.g. also check if there's been a successful response in the last 10 seconds
for a different request, but this is a starting point and a work in progress.
For #26
Fixes #24

The timeout can be passed in as follows from the Provider to the Client: new apn.Provider({..., timeout: 30000}); // raise the request timeout from 10 seconds to 30 seconds

[davimacedo]

@funkenstrahlen thoughts here?

[davimacedo]

It looks good for me. Let's just wait if @funkenstrahlen has any feedback and we can merge.

[TysonAndre]

There's probably a better way to do this, e.g. also check if there's been a successful response in the last 10 seconds
for a different request, but this is a starting point and a work in progress.

I got around to implementing that and added a test. @davimacedo

If, for example, someone were sending thousands of push requests to APNs over dialup quality internet, this PR would probably lead to worse throughput without the latest commit due to constantly destroying and creating connections that were working but limited by reasons other than APNs having silently disconnected

[TysonAndre]

I don't have any other pending changes at the moment unless I see unexpected issues during real usage

[TysonAndre]

I've been running into issues with http2 and aborting requests due to goaway frames, possibly related to multiple goaway frames being sent at once, which may be something that may or may not only occur in these unit tests with the test http server I've implemented, not in APNs.

• I'm not sure if the crash is the same place where the preconditions started failling to hold - I'm not experienced with debugging node
• I'm not sure if this is somehow related to creating a server and tearing it down on the same port, or the http:// insecure http2
• Dropping node 8 may make sense if this is a concern.
• I've seen different crashes in Node v12.18.3 before refactoring this

This time, it was reproduced in travis, but only for node 8 so far, https://travis-ci.com/github/parse-community/node-apn/jobs/394316640

https://nodejs.org/en/about/releases/ indicates that node 10 is the oldest version that's still supported, so this

/home/travis/.nvm/versions/node/v8.17.0/bin/node[5214]: ../src/base-object-inl.h:46:virtual node::BaseObject::~BaseObject(): Assertion `persistent_handle_.IsEmpty()' failed.
 1: node::Abort() [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 2: 0x8cd87b [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 3: 0x8a8b93 [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 4: node::http2::Http2Stream::~Http2Stream() [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 5: node::Environment::RunAndClearNativeImmediates() [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 6: node::Environment::CheckImmediate(uv_check_s*) [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 7: 0x9c1a2c [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 8: uv_run [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
 9: node::Start(uv_loop_s*, int, char const* const*, int, char const* const*) [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
10: node::Start(int, char**) [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
11: __libc_start_main [/lib/x86_64-linux-gnu/libc.so.6]
12: 0x89f601 [/home/travis/.nvm/versions/node/v8.17.0/bin/node]
Aborted (core dumped)

[davimacedo]

I've inspired in your PR and created a new one: #29

[TysonAndre]

Closing in favor of #29